This invention relates to manufacturing of high-gloss high quality magazine paper grades by using on-line calendering. In on-line calendering the calender is arranged directly after a paper machine or a coater and the web is also led directly to the calender. This kind of calendered paper grades have previously been produced by off-line calenders and normally two or three calenders have been used for handling paper produced on one production line and the paper has been rolled before calendering. The speed of prior supercalenders has limited their use as on-line calenders. However, today's development of modern supercalenders and new multi-nip calenders has made it possible to increase the operational speed of these calenders to the level of the production speed of paper machines and coaters, which has made it possible to use these calenders also in on-line configurations. All multi-nip calenders comprise several nips formed of soft and hard rolls. The outer surface of the soft rolls is made of paper or other tightly pressed fiber material or a suitable polymer material. The hard rolls are made of steel, and most of the hard rolls can be heated by oil, water steam or by other means, like electric induction.
The purpose of the calendering is to increase the smoothness, gloss and other properties of the printing surface of the paper. These improved properties of the printing surface improve the final quality of the printed sheet. The printability of paper and the quality of the printed surface are primary quality factors that are valued by the users of paper.
The smoothing of the paper surface is achieved by simultaneously subjecting the fiber structure to high pressure and heat by heating the hard rolls and pressing the rolls together so that a high linear nip force is created on the nips between the rolls. Under the influence of these forces the fibers forming the paper reach their glass transit temperature, and the deformation caused by the nip load is permanent. Sliding of the paper surface on the surface of the rolls may also cause deformation of the fibers and increase the smoothing effect.
When multi-nip calendering has been used, the paper has been traditionally produced in a paper machine and subsequently coated if so desired. In both cases the coated or uncoated paper has been rolled on storage rolls and calendered in separate calenders. The paper has been dried to a very low moisture, typically to 1-3% calculated from total weight of the paper. Before calendering, the paper is rewetted up to a higher moisture content required for good calendering results, typically to 6-10% calculated from the total weight of the paper. The reason for drying to very low moisture content is to level out the cross machine (CD) moisture profile. The short storage on storage rolls before calendering also evens out the moisture of the paper on the roll as well as rewetting before calendering. In present on-line calendering concepts, the paper is dried to a very low moisture content before calendering and wetted just before calendering. The process is therefore almost the same as on off-line calendering, only without moisture settling storage.
The rewetting can be done, for example, with the a water spray application units described in the U.S. Pat. No. 5,286,348 which is incorporated herein by reference, which describes a rewetting apparatus for providing a good moisture profile in the CD direction.
The problem associated the drying and subsequent rewetting of the paper is the time needed for paper to absorb the water and the moisture to even out, especially in the direction of the thickness of the paper and over the surface area of the web. If the rewetting is done just before the calendering, the uneven moisture profile will effect the final surface properties of the produced paper, and the quality grading of the paper is lowered. As stated above, the paper rolls have been reeled up after rewetting and transferred to a waiting station for moisture equalization, whereafter the rolls have been brought to off-line calenders for final calendering for producing a high gloss and to densify the surface of the paper. In these off-line systems, no need for improvement in moisture control was needed, because traditionally supercalendered paper grades like SCA and LWC were calendered in off-line calenders at lower speeds than the speed of the paper machine and there was enough time for settling of the moisture on the storage rolls.
The drying and rewetting process adds to energy consumption required for paper making and to the required space compared to a process where there is no rewetting and overdrying before calendering. Uneven moisture profile results in uneven gloss and uneven thickness profiles because of the effect of the moisture to the fiber deformability. If the thickness profile is uneven, it results in difficulties in winding and may even cause cross-directional bumps in the client or customer rolls. The CD-bumps decrease the runnability of the paper in printing presses and converting machines and by this way decrease the quality of the material from the customer's point of view .
The moisture profile effects many factors of the paper making process and properties of the paper. One very notable feature is that when moisture profile differences are present in the paper, the dryer parts of the paper start shrinking earlier and they shrink more than the wet parts, which leads to stretching of the wet parts. The uneven stretching leads to shrinking of the dry parts and stretching of the wet parts, which further leads to thickness variation, variation in shrinkage and variation of the properties of the paper. A more detailed description of the effect of the moisture and moisture variations is presented with the detailed description of the invention.
The moisture profile of the paper that is produced is controlled presently in several ways, especially at the beginning of the web formation. The purpose of the control of the moisture profile in present technology is to ensure good runnability of the machine and the product that is produced. This is understandable, since there is a strong relationship between the moisture profile and the tension profile. In off-line calendering, the moisture profile is preferably kept as even as possible in those parts of the manufacturing process where the effect of the tension profile is highest on the runnability. The tensions induced into the web by moisture variations and the tension profile do not effect the properties of the final product as such since the tensions have time to relaxate during the waiting or storage time before calendering. Normal waiting time in a modern paper mill producing off-line calendered paper is about 1-5 hours. Present moisture control methods to do take into account the requirements of multinip on-line calendering, and therefore the quality of the calendered paper may even be adversely affected by present moisture control procedures.